The present invention relates to a translucent screen onto which an image light emitted from an image source is projected, and a rear projection display apparatus including the translucent screen.
FIG. 14 is a vertical cross-sectional view schematically showing main components of a rear projection display apparatus incorporating a conventional translucent screen, and FIG. 15 is an enlarged horizontal cross-sectional view taken along a line S15xe2x80x94S15 of FIG. 14. In these figures, a reference numeral 1 denotes an image source such as a video projector, and a reference numeral 2 denotes a translucent screen which transmits the image light emitted from the image source 1 and brings the image light into focus to form an image. Further, a reference numeral 3 denotes a Fresnel lens sheet, and a reference numeral 4 denotes a lenticular lens sheet. The Fresnel lens sheet 3 and the lenticular lens sheet 4 compose the translucent screen 2. A reference numeral 5 denotes a frame member that integrally holds the translucent screen 2.
In the rear projection display apparatus having the conventional translucent screen 2, when the image light is emitted from the image source 1, the light beams are diverged and projected onto the translucent screen 2. When the light beams reach the Fresnel lens sheet 3 as diverging light, the light beams are refracted to substantially parallel beams, as shown in FIG. 15, then impinge on the lenticular lens sheet 4, and are brought into focus to form an image. The light beams from the lenticular lens sheet 4 suitably diverge in the horizontal and vertical directions and are perceived as an image by a viewer facing the screen 2.
The conventional translucent screen 2 is a combination of the Fresnel lens sheet 3 and the lenticular lens sheet 4, and is securely held by the frame member 5 such as a frame member secured on the cabinet or a part of the cabinet of the rear projection display apparatus.
Both the Fresnel lens sheet 3 and the lenticular lens sheet 4 are usually made of an acrylic resin, polycarbonate resin, or the like, which has a high light transmission, appropriate rigidity, and capability of volume production.
In the rear projection display apparatus having the conventional translucent screen 2, which is configured as described above, if both or either of the Fresnel lens sheet 3 or the lenticular lens sheet 4 forming the screen 2 is thin, the two sheets 3 and 4 are not properly cemented, causing a floating (hereafter referred to as a separation) of a part or entirety of the screen 2 because of the reasons described below.
The separation in the screen 2 maybe found, when the sheet 3 or 4 is warped in the sheet forming process prior to screen assembly and the warp does not disappear after the screen assembly. Possible causes of the separation are variations or the like in the sheet forming conditions which cause an internal stress or distortion of the sheet 3 or 4 in the sheet forming process, resulting in a warp of the sheet 3 or 4 at the end of the sheet forming process. Other possible causes of the warp of the separation are variations in the environment such as ambient temperature and humidity from after the sheet forming process to the screen assembly. The separation may also occur after the screen is assembled. This separation is caused by variations or the like in the environment in which the projection display apparatus is placed after the translucent screen 2 is assembled, more specifically, a difference in expansion coefficient between the two sheets 3 and 4, which causes the sheets 3 and 4 to expand or contract by different amounts. These phenomena are clearly found especially if the sheets 3 and 4 are thin and have a low rigidity.
The translucent screen 2 is designed so that an optimum image is projected when the screen 2 is placed in a certain position referred to the image source 1. The translucent screen 2 formed by two or more sheets is designed so that an optimum image is projected when the sheets 3 and 4 are tightly joined one another or when the sheets 3 and 4 are placed in certain positions referred to one another. If a separation occurs, the fundamental functions of the screen 2 are adversely affected, causing the projected image to have a deteriorated resolution or partly colored. Some causes of the problems lie in the initial performance of the translucent screen 2 while others in variations with time of the performance of the translucent screen 2 after the screen 2 is assembled, as described above.
The conventional solutions to the problems have been the two methods described below.
A first method is to form a thick Fresnel lens sheet 3 and/or a lenticular lens sheet 4. A second method is to dispose a flat translucent plate such as an acrylic plate or a glass plate on the side of the viewer of the lenticular lens sheet 4, as described in Japanese Utility Model Kokai Publication No. 04-129150 published on Nov. 25, 1992 in order to provide an appropriate rigidity to the whole translucent screen 2.
The first method of forming the thick Fresnel lens sheet 3 and/or the lenticular lens sheet 4, however, lowers the degree of flexibility in lens design of the Fresnel lens sheet 3 and/or the lenticular lens sheet 4. Accordingly, the translucent screen 2 cannot provide desired performance, or the effect of stray light generated in the translucent screen 2 may increase to greatly degrade the quality of the projected image.
First, the relationship between the thickness of the screen 2 and the effect of the stray light on the image will next be described.
FIG. 16 and FIG. 17 illustrate differences in the effect of stray light on the image. Both figures are partially enlarged vertical cross-sectional view of the translucent screen 2 indicated in FIG. 14. FIG. 16 shows a screen in which the Fresnel lens sheet 3 is thicker than the lenticular lens sheet 4. FIG. 17 shows a screen in which the Fresnel lens sheet 3 is relatively thin. In the figures, solid-line arrows indicate a part of the trace of the image light, and a broken-line arrow indicates a part of the trace of the stray light generated in the interface between the Fresnel lens sheet 3 and the air on the side of the light outgoing surface. Just an example of stray light is shown in the figure. Many other types of stray light can be generated in the translucent screen 2.
The stray light is reflected again in the interface between the Fresnel lens sheet 3 and the air on the side of the light incoming surface, passes the light outgoing surface of the Fresnel lens sheet 3 and the lenticular lens sheet 4, and reaches a viewer. If the Fresnel lens sheet 3 is thick as shown in FIG. 16, a path of the stray light is very far from a path of the original image light, significantly degrading the resolution and contrast of the image. If Fresnel lens sheet 3 is thin, a path of the stray is near to a path of the original image light, as shown in FIG. 17. Accordingly, as the thickness of the Fresnel lens sheet 3 decreases, the degradation of image light caused by the stray light is reduced.
In the second method of giving an appropriate rigidity to the whole translucent screen 2, a flat translucent plate or the like is disposed on the side of the viewer of the lenticular lens sheet 4. This method, however, poses the attenuation of the image light passing the flat translucent plate. In addition, as described with reference to FIG. 16, the light reflected in the interface between the flat translucent plate and the air causes stray light to be generated inside the flat translucent plate, significantly degrading the image light.
Either of these two approaches to get rid of the separation in the rear projection display apparatus having the conventional translucent screen cannot securely prevent image degradation and other problems from occurring.
It is an object of the present invention to provide a translucent screen that prevents separations, minimizing degradation in image projected thereon, and a rear projection display apparatus including the translucent screen.
According to an aspect of the present invention, a translucent screen onto which an image light emitted from an image source is projected, comprises: a first screen member that expands and contracts with variations in ambient temperature and humidity; and a second screen member that expands and contracts with variations in ambient temperature and humidity, the second screen member having a relatively lower rigidity than the first screen member; wherein the second screen member is secured on the first screen member so that the second screen member is held under tension by the first screen member at the normal ambient temperature and humidity.
The second screen member may be expanded at a high ambient temperature which is higher than the normal ambient temperature and/or a high ambient humidity which is higher than the normal ambient humidity, immediately before the second screen member is secured on the first screen member.
Further, the first screen member may is contracted at a low ambient temperature which is lower than the normal ambient temperature and/or a low ambient humidity which is lower than the normal ambient humidity, immediately before the second screen member is secured on the first screen member.
Furthermore, the second screen member may be expanded at a high ambient temperature which is higher than the normal ambient temperature and/or a high ambient humidity which is higher than the normal ambient humidity, and the first screen member may be contracted at a low ambient temperature which is lower than the normal ambient temperature and/or a low ambient humidity which is lower than the normal ambient humidity, immediately before the second screen member is secured on the first screen member.
Moreover, the first screen member has a first expansion coefficient, the second screen member has a second expansion coefficient which is larger than the first expansion coefficient, and the first screen member and the second screen member are placed at a high ambient temperature which is higher than the normal ambient temperature and/or a high ambient humidity which is higher than the normal ambient humidity when the second screen member is secured on the first screen member.
Further, the first screen member has a first expansion coefficient, the second screen member has a second expansion coefficient which is smaller than the first expansion coefficient, and the first screen member and the second screen member are placed at a low ambient temperature which is lower than the normal ambient temperature and/or a low ambient humidity which is lower than the normal ambient humidity when the second screen member is secured on the first screen member.
According to another aspect of the present invention, a rear projection display apparatus comprises a translucent screen onto which an image light emitted from an image source is projected, the translucent screen expanding and contracting with variations in ambient temperature and humidity; and a frame member for holding the translucent screen, the frame member having a relatively higher rigidity than the translucent screen; wherein the translucent screen is secured on the frame member so that the translucent screen is held under tension by the frame member at the normal ambient temperature and humidity.
Further, the translucent screen may be expanded at a high ambient temperature which is higher than the normal ambient temperature and/or a high ambient humidity which is higher than the normal ambient humidity, immediately before the translucent screen is secured on the frame member.
Furthermore, the frame member may be contracted at a low ambient temperature which is lower than the normal ambient temperature, immediately before the translucent screen is secured on the frame member.
Moreover, the translucent screen may be expanded at a high ambient temperature which is higher than the normal ambient temperature and/or a high ambient humidity which is higher than the normal ambient humidity, and the frame member may be contracted at a low ambient temperature which is lower than the normal ambient temperature, immediately before the translucent screen is secured on the frame member.
Additionally, the translucent screen has a first expansion coefficient, the frame member has a second expansion coefficient which is smaller than the first expansion coefficient, and the translucent screen and the frame member are placed at a high ambient temperature which is higher than the normal ambient temperature and/or a high ambient humidity which is higher than the normal ambient humidity when the translucent screen is secured on the frame member.
Further, the translucent screen has a first expansion coefficient, the frame member has a second expansion coefficient which is larger than the first expansion coefficient, and the translucent screen and the frame member are placed at a low ambient temperature which is lower than the normal ambient temperature and/or a low ambient humidity which is lower than the normal ambient humidity when the translucent screen is secured on the frame member.